Declare platform.h functions with C linkage (earlephilhower#2218)

The platform.h headers for both RP2040 and RP2350 need 'extern "C"'
declarations when included from C++ code or 'rp2040_chip_version'
and 'rp2350_chip_version' won't be found by the linker.

Author: rhulme

src/rp2040/pico_platform/include/pico/platform.h

@@ -77,6 +77,10 @@
 
 #ifndef __ASSEMBLER__
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*! \brief No-op function for the body of tight loops
  *  \ingroup pico_platform
  *
@@ -210,6 +214,10 @@ return a;
     (__builtin_popcount(b) >= 2 ? __mul_instruction(a,b) : (a)*(b)), \
     (a)*(b))
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // __ASSEMBLER__
 
 #endif

src/rp2350/pico_platform/include/pico/platform.h

@@ -70,6 +70,10 @@
 
 #ifndef __ASSEMBLER__
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*! \brief No-op function for the body of tight loops
  *  \ingroup pico_platform
  *
@@ -281,6 +285,10 @@ __force_inline static int32_t __mul_instruction(int32_t a, int32_t b) {
     (__builtin_popcount(b) >= 2 ? __mul_instruction(a,b) : (a)*(b)), \
     (a)*(b))
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // __ASSEMBLER__
 
 #endif