package/libmicrohttpd: defining _REENTRANT on ARC is no longer needed

The ARC gcc compiler now defines _REENTRANT when -pthread is
passed. This issue was fixed upstream in gcc commit
de4c7f60f2891193bf3f5da823b17fa0d7fd4830, which is part of gcc
7.x. Therefore, both the gcc 7.x and 8.x versions, which can be
selected for the ARC architecture, are fixed, making the libmicrohttpd
work around useless.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>