RISC-V/GAS: Correct an `expr' global shadowing error for pre-4.8 GCC

RISC-V/GAS: Correct an `expr' global shadowing error for pre-4.8 GCC

Correct a commit f0531ed6a429 ("Compress loads/stores with implicit 0
offset.") regression and remove a `-Wshadow' compilation error:

cc1: warnings being treated as errors
.../gas/config/tc-riscv.c: In function 'riscv_handle_implicit_zero_offset':
.../gas/config/tc-riscv.c:1194: error: declaration of 'expr' shadows a global declaration
.../gas/expr.h:180: error: shadowed declaration is here
make[4]: *** [tc-riscv.o] Error 1

which for versions of GCC before 4.8 prevents GAS for RISC-V targets
from being built.  See also GCC PR c/53066.

	gas/
	* config/tc-riscv.c (riscv_handle_implicit_zero_offset): Rename
	`expr' parameter to `ep'.