[X86] Fix variadic argument handling for x32
The X86-64 ABI defines va_list as
typedef struct {
unsigned int gp_offset;
unsigned int fp_offset;
void *overflow_arg_area;
void *reg_save_area;
} va_list[1];
This means the size, alignment, and reg_save_area offset will depend on
whether we are in LP64 or in ILP32 mode, so this commit adds the checks.
Additionally, the VAARG_64 pseudo-instruction assumed 64-bit pointers, so
this commit adds a VAARG_X32 pseudo-instruction that behaves just like
VAARG_64, except for assuming 32-bit pointers.
Some of these changes were originally done by
Michael Liao <michael.hliao@gmail.com>.
Fixes https://bugs.llvm.org/show_bug.cgi?id=48428.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D93160