[compiler-rt] Add platform detection support for x32

Currently, the compiler-rt build system checks only whether __X86_64
is defined to determine whether the default compiler-rt target arch
is x86_64. Since x32 defines __X86_64 as well, we must also check that
the default pointer size is eight bytes and not four bytes to properly
detect a 64-bit x86_64 compiler-rt default target arch.

Reviewed By: hvdijk, vitalybuka

Differential Revision: https://reviews.llvm.org/D99988