eal: fix alarm clock type for glibc < 2.12

eal: fix alarm clock type for glibc < 2.12

CLOCK_MONOTONIC_RAW added in glibc 2.12, using this define in older
glibc versions cause compile error:
'error: identifier "CLOCK_MONOTONIC_RAW" is undefined'

This patch replaces "CLOCK_MONOTONIC_RAW" with "CLOCK_MONOTONIC" for
older glibc versions, versions that support "CLOCK_MONOTONIC_RAW"
will keep using this clock type.

Fixes: d08d304508a8 ("eal/linux: make alarm not affected by system time jump")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>