[libc] Enable aliasing on AMDGPU targets

AMDGPU supports aliases now, so we can drop this case and leave it only
for the NVPTX target. Unfortunately it's unlikely that NVPTX will be
able to support this in the future due to their PTX language being very
limited.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D154704

Author: jhuber6

libc/src/__support/common.h

@@ -16,8 +16,8 @@
 #define LLVM_LIBC_FUNCTION_ATTR
 #endif
 
-// GPU targets do not support aliasing.
-#if defined(LIBC_COPT_PUBLIC_PACKAGING) && defined(LIBC_TARGET_ARCH_IS_GPU)
+// The NVPTX target does not support aliasing.
+#if defined(LIBC_COPT_PUBLIC_PACKAGING) && defined(LIBC_TARGET_ARCH_IS_NVPTX)
 #define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)                           \
   LLVM_LIBC_FUNCTION_ATTR decltype(__llvm_libc::name)                          \
       __##name##_impl__ __asm__(#name);                                        \