GCC 6 warns:

error: result of ‘63 << 26’ requires 33 bits to represent, but ‘int’ only has 32 bits [-Werror=shift-overflow=]

on 0x3f << 26.  This patch adds 'U' suffix to make integer constant
unsigned.

	* alpha.c (OP_Jxx): Add 'U' suffix to make it unsigned.
	(OP_BSR): Likewise.
	(Jxx_FUNC_JMP): Likewise.
	(Jxx_FUNC_JSR): Likewise.
	(Jxx_FUNC_RET): Likewise.
	(Jxx_FUNC_JSR_COROUTINE): Likewise.
	(alpha_find_call): Replace 0x3f with 0x3fU.