[crt] Remove __USER_LABEL_PREFIX__

[MaskRay]

Summary

The .init_array code is ELF specific. For ELF platforms,
__USER_LABEL_PREFIX__ is defined as "". Make the simplification
so that downstream ELF targets can build this file even if
__USER_LABEL_PREFIX__ is undefined.

[barannikov88]

Related: D147085
__USER_LABEL_PREFIX__ is always defined. It is usually defined to nothing though, i.e. just
#define __USER_LABEL_PREFIX__
In this case the file compiles just fine. It does not compile if the prefix is non-empty, e.g.:
#define __USER_LABEL_PREFIX__ _
because it is just a token and not a string.
I don't know if it is valid for the label prefix to be non-empty on ELF platforms (and couldn't find an example),
but I don't see why it couldn't be.

OTOH, for in-tree targets it should be safe to drop the prefix as it always expands to nothing.
Out-of-tree targets may retain it as necessary under target-specific #ifdef.

I'm OK with this change with one nit: could you run clang-format on the changed lines?

compiler-rt/lib/crt/crtbegin.c

 #endif
 }
 
 #ifdef CRT_HAS_INITFINI_ARRAY
 __attribute__((section(".init_array"),
                used)) static void (*__init)(void) = __do_init;
 #elif defined(__i386__) || defined(__x86_64__)
 __asm__(".pushsection .init,\"ax\",@progbits\n\t"
-    "call " __USER_LABEL_PREFIX__ "__do_init\n\t"
+        "call __do_init\n\t"
         ".popsection");
 #elif defined(__riscv)
 __asm__(".pushsection .init,\"ax\",%progbits\n\t"
-        "call " __USER_LABEL_PREFIX__ "__do_init\n\t"
+        "call __do_init\n\t"
         ".popsection");
 #elif defined(__arm__) || defined(__aarch64__)
 __asm__(".pushsection .init,\"ax\",%progbits\n\t"
-    "bl " __USER_LABEL_PREFIX__ "__do_init\n\t"
+        "bl __do_init\n\t"
         ".popsection");
 #elif defined(__powerpc__) || defined(__powerpc64__)
 __asm__(".pushsection .init,\"ax\",@progbits\n\t"
-    "bl " __USER_LABEL_PREFIX__ "__do_init\n\t"
+        "bl __do_init\n\t"
         "nop\n\t"
         ".popsection");
 #elif defined(__sparc__)
 __asm__(".pushsection .init,\"ax\",@progbits\n\t"
-    "call " __USER_LABEL_PREFIX__ "__do_init\n\t"
+        "call __do_init\n\t"
         ".popsection");
 #else
 #error "crtbegin without .init_fini array unimplemented for this architecture"
 #endif // CRT_HAS_INITFINI_ARRAY
 
 #ifndef CRT_HAS_INITFINI_ARRAY
 static fp __DTOR_LIST__[]
     __attribute__((section(".dtors"), aligned(sizeof(fp)))) = {(fp)-1};
 extern fp __DTOR_LIST_END__[];

 #endif
 }
 
 #ifdef CRT_HAS_INITFINI_ARRAY
 __attribute__((section(".fini_array"),
                used)) static void (*__fini)(void) = __do_fini;
 #elif defined(__i386__) || defined(__x86_64__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
-    "call " __USER_LABEL_PREFIX__ "__do_fini\n\t"
+        "call __do_fini\n\t"
         ".popsection");
 #elif defined(__arm__) || defined(__aarch64__)
 __asm__(".pushsection .fini,\"ax\",%progbits\n\t"
-    "bl " __USER_LABEL_PREFIX__ "__do_fini\n\t"
+        "bl __do_fini\n\t"
         ".popsection");
 #elif defined(__powerpc__) || defined(__powerpc64__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
-    "bl " __USER_LABEL_PREFIX__ "__do_fini\n\t"
+        "bl __do_fini\n\t"
         "nop\n\t"
         ".popsection");
 #elif defined(__riscv)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
-        "call " __USER_LABEL_PREFIX__ "__do_fini\n\t"
+        "call __do_fini\n\t"
         ".popsection");
 #elif defined(__sparc__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
-    "call " __USER_LABEL_PREFIX__ "__do_fini\n\t"
+        "call __do_fini\n\t"
         ".popsection");
 #else
 #error "crtbegin without .init_fini array unimplemented for this architecture"
 #endif  // CRT_HAS_INIT_FINI_ARRAY