[nrf fromtree] picolibc: Update module to version 1.8.8

Switch to released version with all of the Zephyr build fixes

Signed-off-by: Keith Packard <[email protected]>
(cherry picked from commit d5b6539)

1.8.8 came just 2 weeks after 1.8.7 but includes several bugfixes,
including a fix to a bug that prevented building with gcc9 which is the
default in Ubuntu 20.04.
https://github.com/picolibc/picolibc/releases/tag/1.8.8

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: keith-packard

west.yml

@@ -315,7 +315,7 @@ manifest:
         - debug
     - name: picolibc
       path: modules/lib/picolibc
-      revision: 06bde1fd7531b1f788f6e42b6f7b358c0fe4f814
+      revision: e15656f7682500fc8953e1a2008f7b1ae4f65ce8
     - name: segger
       revision: b011c45b585e097d95d9cf93edf4f2e01588d3cd
       path: modules/debug/segger