[WebAssembly] Add target specific overrides for lgamma family functions

[jgravelle-google]

Summary

Current WebAssembly implementations use musl libc, which writes to
signgam in its lgamma functions. We want the rest of the builtins to be
const under -fno-math-errno, so overwrite lgamma's builtin definitions
to be never-const.

[efriedma]

It doesn't make sense to do this in WebAssembly-specific code; every POSIX platform has a signgam which behaves the same way.

[jgravelle-google]

https://clang.llvm.org/compatibility.html doesn't mention anything about POSIX, only C11 compliance, so I didn't think Clang in general cared about POSIX.
That being said I can definitely agree that Clang shouldn't preclude POSIX. I'll open a more-general diff.

include/clang/Basic/BuiltinsWebAssembly.def

 /// \brief This file defines the WebAssembly-specific builtin function database.
 /// Users of this file must define the BUILTIN macro to make use of this
 /// information.
 ///
 //===----------------------------------------------------------------------===//
 
 // The format of this database matches clang/Basic/Builtins.def.
 
+#if defined(BUILTIN) && !defined(LIBBUILTIN)
+#  define LIBBUILTIN(ID, TYPE, ATTRS, HEADER) BUILTIN(ID, TYPE, ATTRS)
+#endif
+
 // Note that current_memory is not "c" (readnone) because it must be sequenced
 // with respect to grow_memory calls.
 BUILTIN(__builtin_wasm_current_memory, "z", "n")
 BUILTIN(__builtin_wasm_grow_memory, "zz", "n")
 
+// Redeclaring lgamma from Builtins.def as non-const to allow for POSIX
+// compatibility. Specifically lgamma writes to a user-visible global, signgam,
+// that is available in POSIX-compliant libcs.
+LIBBUILTIN(lgamma, "dd", "fn", "math.h")
+LIBBUILTIN(lgammaf, "ff", "fn", "math.h")
+LIBBUILTIN(lgammal, "LdLd", "fn", "math.h")
+
 #undef BUILTIN
+#undef LIBBUILTIN