Add hardware_exception for setting exception handlers at runtime (#380)

src/rp2_common/pico_standard_link/crt0.S

@@ -74,6 +74,11 @@ __vectors:
 .word isr_irq30
 .word isr_irq31
 
+// all default exception handlers do nothing, and we can check for them being set to our
+// default values by them pointing to between __defaults_isrs_start and __default_isrs_end
+.global __default_isrs_start
+__default_isrs_start:
+
 // Declare a weak symbol for each ISR.
 // By default, they will fall through to the undefined IRQ handler below (breakpoint),
 // but can be overridden by C functions with correct name.
@@ -94,6 +99,9 @@ decl_isr_bkpt isr_svcall
 decl_isr_bkpt isr_pendsv
 decl_isr_bkpt isr_systick
 
+.global __default_isrs_end
+__default_isrs_end:
+
 .macro decl_isr name
 .weak \name
 .type \name,%function