arm: mxs: make inline function compatible for GCC 5

arm: mxs: make inline function compatible for GCC 5

GCC 5 uses C99 inline semantics. To generate external visibility in C99 for
the lowlevel_init() function must be declared with extern inline [1].

The GNU89 inline semantic is the same as C99 extern inline.

Use the __GNUC_STDC_INLINE__ macro to detect if C99 inline semantic will be
used by GCC.

Fixes the build error "undefined reference to `lowlevel_init'" for mx28 based
targets.

[1] "Different semantics for inline functions"
https://gcc.gnu.org/gcc-5/porting_to.html

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>