[Core] Fix heap symbols and syscalls for picolibc

picolibc internally uses __heap_start and __heap_end instead of the
defacto chibios linker script standard __heap_base__ and __heap_end__
therefore we introduce these symbols as an alias. Usually all memory
used within QMK is statically allocated, but some algorithms make usage
of malloc and friends.

Also the timeval struct is not defined by picolibc for syscalls, therefore it
is declared as stub.

platforms/chibios/platform.mk

@@ -316,7 +316,7 @@ endif
 #
 
 # Use defined stack sizes of the main thread in linker scripts
-LDSYMBOLS =--defsym=__process_stack_size__=$(USE_PROCESS_STACKSIZE),--defsym=__main_stack_size__=$(USE_EXCEPTIONS_STACKSIZE)
+SHARED_LDSYMBOLS = -Wl,--defsym=__process_stack_size__=$(USE_PROCESS_STACKSIZE),--defsym=__main_stack_size__=$(USE_EXCEPTIONS_STACKSIZE)
 
 # Shared Compiler flags for all toolchains
 SHARED_CFLAGS = -fomit-frame-pointer \
@@ -327,7 +327,6 @@ SHARED_CFLAGS = -fomit-frame-pointer \
 
 # Shared Linker flags for all toolchains
 SHARED_LDFLAGS = -T $(LDSCRIPT) \
-                 -Wl,$(LDSYMBOLS) \
                  -Wl,--gc-sections \
                  -nostartfiles
 
@@ -349,11 +348,15 @@ ifeq ($(strip $(MCU)), risc-v)
     # Default to compiling with picolibc for RISC-V targets if available, which
     # is available by default on distributions based on Debian 11+.
     ifeq ($(shell $(TOOLCHAIN)gcc --specs=picolibc.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
-        # Toolchain specific Compiler flags
-        # Note that we still link with our own linker script
-        # by providing it via the -T flag above.
+        # Toolchain specific Compiler flags Note that we still link with our own
+        # linker script by providing it via the -T flag in SHARED_LDFLAGS.
         TOOLCHAIN_CFLAGS = --specs=picolibc.specs
 
+        # picolibc internally uses __heap_start and __heap_end instead of the
+        # defacto chibios linker script standard __heap_base__ and __heap_end__
+        # therefore we introduce these symbols as an alias.
+        TOOLCHAIN_LDSYMBOLS = -Wl,--defsym=__heap_start=__heap_base__,--defsym=__heap_end=__heap_end__
+
         # Tell QMK that we are compiling with picolibc.
         OPT_DEFS += -DUSE_PICOLIBC
     endif
@@ -404,7 +407,7 @@ CFLAGS   += $(SHARED_CFLAGS) $(TOOLCHAIN_CFLAGS)
 CXXFLAGS += $(CFLAGS) $(SHARED_CXXFLAGS) $(TOOLCHAIN_CXXFLAGS) -fno-rtti
 
 # Linker flags
-LDFLAGS  += $(SHARED_LDFLAGS) $(TOOLCHAIN_LDFLAGS) $(MCUFLAGS)
+LDFLAGS  += $(SHARED_LDFLAGS) $(SHARED_LDSYMBOLS) $(TOOLCHAIN_LDFLAGS) $(TOOLCHAIN_LDSYMBOLS) $(MCUFLAGS)
 
 # Tell QMK that we are hosting it on ChibiOS.
 OPT_DEFS += -DPROTOCOL_CHIBIOS

platforms/chibios/syscall-fallbacks.c

@@ -22,6 +22,7 @@
  * the _reent struct has to be defined. */
 #if defined(USE_PICOLIBC)
 struct _reent;
+struct timeval;
 #endif
 
 #pragma GCC diagnostic ignored "-Wmissing-prototypes"