cmd/compile: track pragmas in lexer rather than global variables

cmd/compile: track pragmas in lexer rather than global variables

By using a Pragma bit set (8 bits) rather than 8 booleans, also
reduce Func type size by 8 bytes (208B -> 200B on 64bit platforms,
116B -> 108B on 32bit platforms).

Change-Id: Ibb7e1f8c418a0b5bc6ff813cbdde7bc6f0013b5a
Reviewed-on: https://go-review.googlesource.com/19966
Reviewed-by: Dave Cheney <dave@cheney.net>