m-profile/gcc: Support picolibc startup mechanism (ARM-software#246)

Use picolibc's _start function instead of providing our own.

Signed-off-by: Keith Packard <[email protected]>
(cherry picked from commit e902f86)

Author: keith-packard

CMSIS/Core/Include/m-profile/cmsis_gcc_m.h

@@ -29,6 +29,10 @@
 
 #include <arm_acle.h>
 
+#if defined(__PICOLIBC__) && !defined(__PROGRAM_START)
+#define __PROGRAM_START _start
+#endif
+
 /* #########################  Startup and Lowlevel Init  ######################## */
 #ifndef __PROGRAM_START