Fix build error in aarch64-linux-tdep.c on macOS

Fix build error in aarch64-linux-tdep.c on macOS

When building with --enable-targets=all on macOS, I get this error:

  CXX    aarch64-linux-tdep.o
/Users/simark/src/binutils-gdb/gdb/aarch64-linux-tdep.c:328:7: error: no matching function for call to 'store_integer'
      store_integer ((gdb_byte *)&vg_target, sizeof (uint64_t), byte_order,
      ^~~~~~~~~~~~~
/Users/simark/src/binutils-gdb/gdb/defs.h:556:13: note: candidate template ignored: requirement 'Or<is_same<unsigned long long, long>, is_same<unsigned long long, unsigned long> >::value' was not satisfied [with T = unsigned long long]
extern void store_integer (gdb_byte *addr, int len, enum bfd_endian byte_order,
            ^

I believe it's because uint64_t is defined as "unsigned long long" on macOS,
even though "unsigned long" is also 64 bits.  Other 64-bits platforms define
uint64_t as "unsigned long".

This makes the type of the argument to store_integer (unsigned long long) not
match the requirement that it must be the same as ULONGEST, which is unsigned
long.

Fix it by changing the type of the vl variable to be ULONGEST, which is what
extract_unsigned_integer returns anyway.

gdb/ChangeLog:

	* aarch64-linux-tdep.c (aarch64_linux_supply_sve_regset): Change type
	of vl to ULONGEST.