[AMDGPU][llvm-mc] Predefined symbols to access register counts (.kernel.{v|s}gpr_count)

The feature allows for conditional assembly, filling the entries
of .amd_kernel_code_t etc.

Symbols are defined with value 0 at the beginning of each kernel scope.
After each register usage, the respective symbol is set to:
	value = max( value, ( register index + 1 ) )
Thus, at the end of scope the value represents a count of used registers.

Kernel scopes begin at .amdgpu_hsa_kernel directive, end at the
next .amdgpu_hsa_kernel (or EOF, whichever comes first). There is also
dummy scope that lies from the beginning of source file til the
first .amdgpu_hsa_kernel.

Test added.

Differential Revision: https://reviews.llvm.org/D27859

llvm-svn: 290608