target-xtensa: fix missing errno codes for mingw32

Put the following errno value mappings under #ifdef:

xtensa-semi.c: In function 'errno_h2g':
xtensa-semi.c:113: error: 'ENOTBLK' undeclared (first use in this function)
xtensa-semi.c:113: error: (Each undeclared identifier is reported only once
xtensa-semi.c:113: error: for each function it appears in.)
xtensa-semi.c:113: error: array index in initializer not of integer type
xtensa-semi.c:113: error: (near initialization for 'guest_errno')
xtensa-semi.c:124: error: 'ETXTBSY' undeclared (first use in this function)
xtensa-semi.c:124: error: array index in initializer not of integer type
xtensa-semi.c:124: error: (near initialization for 'guest_errno')
xtensa-semi.c:134: error: 'ELOOP' undeclared (first use in this function)
xtensa-semi.c:134: error: array index in initializer not of integer type
xtensa-semi.c:134: error: (near initialization for 'guest_errno')

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
