[OpenMP][libomp] Fix register constraint for tpause and umwait

[jlpeyton]

Summary

Register constraint switched to "=q" which means very specifically (from https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints)

Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 64-bit mode, any integer register."

Older gcc versions (8.x and below) were trying to use esi or edi for the 8 bit flag variable, but it wound up displaying this error in the end:

kmp_lock.cpp: In function ‘void __kmp_spin_backoff(kmp_backoff_t*)’:
kmp_lock.cpp:2684:1: error: unsupported size for integer register

Hence the correct restriction is "=q" instead of "=r".

Fixes: https://github.com/llvm/llvm-project/issues/53309

[tlwilmar]

Looks good to me.

openmp/runtime/src/kmp.h

 KMP_ATTRIBUTE_TARGET_WAITPKG
 static inline int __kmp_tpause(uint32_t hint, uint64_t counter) {
 #if !KMP_HAVE_WAITPKG_INTRINSICS
   uint32_t timeHi = uint32_t(counter >> 32);
   uint32_t timeLo = uint32_t(counter & 0xffffffff);
   char flag;
   __asm__ volatile("#tpause\n.byte 0x66, 0x0F, 0xAE, 0xF1\n"
                    "setb   %0"
-                   : "=r"(flag)
+                   // The "=q" restraint means any register accessible as rl
+                   //   in 32-bit mode: a, b, c, and d;
+                   //   in 64-bit mode: any integer register
+                   : "=q"(flag)
                    : "a"(timeLo), "d"(timeHi), "c"(hint)
                    :);
   return flag;
 #else
   return _tpause(hint, counter);
 #endif
 }
 KMP_ATTRIBUTE_TARGET_WAITPKG

 KMP_ATTRIBUTE_TARGET_WAITPKG
 static inline int __kmp_umwait(uint32_t hint, uint64_t counter) {
 #if !KMP_HAVE_WAITPKG_INTRINSICS
   uint32_t timeHi = uint32_t(counter >> 32);
   uint32_t timeLo = uint32_t(counter & 0xffffffff);
   char flag;
   __asm__ volatile("#umwait\n.byte 0xF2, 0x0F, 0xAE, 0xF1\n"
                    "setb   %0"
-                   : "=r"(flag)
+                   // The "=q" restraint means any register accessible as rl
+                   //   in 32-bit mode: a, b, c, and d;
+                   //   in 64-bit mode: any integer register
+                   : "=q"(flag)
                    : "a"(timeLo), "d"(timeHi), "c"(hint)
                    :);
   return flag;
 #else
   return _umwait(hint, counter);
 #endif
 }
 #elif KMP_HAVE_MWAIT