site/common-musl: Set ac_cv_sys_file_offset_bits default to 64

off_t is always 64bit on musl regardless of 32bit or 64bit
architectures. autoconf has AC_SYS_LARGEFILE to detect correct off_t
size but it only work with glibc since it defines feature macros
_FILE_OFFSET_BITS=64 and _LARGEFILE_SOURCE but these macros are
not used on musl headers.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>