package/gcc: define _REENTRANT for RISC-V when -pthread is passed

The detection of pthread support fails on RISC-V unless _REENTRANT is
defined. This commit backports a patch that is already in upstream gcc
8.x to gcc 7.x to define _REENTRANT when -pthread is passed.

This will replace a number of package-specific fixes that have been
introduced to define _REENTRANT.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas: tweak commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>